The present invention relates to a polishing apparatus used in, e.g., chemical-mechanical polishing (CMP).
A technique for planarizing a substrate surface by polishing has been employed in many fields including the semiconductor substrate fabrication process. In recent years, CMP for planarizing the unevenness of a surface, e.g., the unevenness of the surface of an interlevel insulating film, formed during the fabrication by polishing is used in a process of fabricating devices on a semiconductor substrate.
In CMP, hard polishing cloth made of a material such as foamed polyurethane, different from relatively soft polishing cloth comprised of unwoven fabric used for polishing the surface of the semiconductor substrate, is used to planarize the insulating film. To obtain the polishing uniformity within the substrate surface, an elastic cushion layer is generally formed under a hard pad.
FIGS. 4A and 4B show the arrangement of a conventional polishing apparatus.
As shown in FIG. 4A, the conventional polishing apparatus is constituted by a substrate holder 409 for holding a polishing target, a polishing table 410 to which a polishing pad 402 is adhered, an abrasive supply member 411, and a conditioning mechanism 413 on which a diamond pellet 412 is mounted. Mechanisms provided to the substrate holder 409 and conditioning mechanism 413 to rotate, swing, and press them, and a rotational mechanism provided to the polishing table 410 are not illustrated.
As shown in FIG. 4B, a retainer ring 401 is set on a surface of the substrate holder 409 which opposes a substrate 405, to correspond to the circumference of the substrate 405. The retainer ring 401 holds the substrate 405 and prevents lateral shift of the substrate 405. As the material of the retainer ring 401, a hard plastic such as polyethylene terephthalate is used. An air cushion 407 applies a downward load to the retainer ring 401. An elastic layer called an insert pad 403 is formed on the surface of the substrate holder 409 inside the retainer ring 401.
By using the polishing arrangement having the above arrangement, for example, the surface of an interlevel insulating layer in the multilevel interconnection structure of an LSI is planarized.
During polishing, the retainer ring 401 prevents not only lateral shift of the substrate 405 but also abnormal polishing of the outer peripheral portion of the substrate 405. More specifically, during polishing, the substrate 405 is urged against the polishing table 410 by the polishing pad 402 consisting of an upper hard layer and a lower soft layer. The contact pressure is the maximum at the outer peripheral portion of the substrate 405.
At this time, as shown in FIG. 5A, the polishing pad 402 is deformed by the pressing force of the substrate 405 for several mm from the outer peripheral portion of the substrate 405, and the pressure acting on the outer peripheral portion of the substrate 405 decreases. As a result, the polishing amount on the outer peripheral portion of the substrate 405 decreases. In particular, depending on the modulus of elasticity of the insert pad 403 and other polishing conditions, a deformation region 501 of the polishing pad 402 sometimes extends for several cm from the outer peripheral portion of the substrate 405.
In the conventional polishing apparatus, abnormal polishing is suppressed in the following manner. First, the surfaces of the retainer ring 401 and substrate 405 that are to come into contact with the polishing pad 402 are set to be flush. The width of the retainer ring 401 with which the retainer ring 401 is to come into contact with the polishing pad 402 is set to be equal to or more than the deformation region described above of the polishing pad 402. This suppresses a deformation region 502 from extending over the outer peripheral portion of the substrate 405, as shown in FIG. 5B.
A load is applied to the retainer ring 401 by the air cushion 407 independently of applying a load to the substrate 405. This makes the pressure that presses the retainer ring 401 against the polishing pad 402 independent and constant. For example, the retainer ring 401 is brought into contact with the polishing pad 402 with a load of about 500 g/cm.sup.2 (.apprxeq.7 psi).
For this reason, during polishing, the retainer ring 401 is also polished by the polishing pad 402, and the material of the retainer ring 401 generated by grinding spreads over the polishing pad 402 as impurities. In this case, if an alloy material such as stainless steel is used to form the retainer ring 401, the metal component generated by grinding spreads over the polishing pad 402 to adversely affect the characteristics of devices formed on the substrate 405. Also, the cutting chips of the alloy material damage the polishing surface of the polishing pad 402. To solve these problems, a plastic is used as the material of the conventional retainer ring 401.
As the process amount increases, the plastic retainer ring 401 deforms, and the specified performance is not maintained.
In this case, even if a hard plastic is used to suppress deformation, its mechanical strength is limited and inferior to that of a metal alloy material such as stainless steel. Even a conventional retainer ring using a hard plastic deforms when the number of polishing processes increases, and the capability of the retainer to press the polishing pad degrades. As a result, in the conventional polishing apparatus, when the number of polishing processes increases, an abnormality in polishing amount occurs on the outer peripheral portion of the substrate as a polishing target.